1. Field of the Invention
The present invention relates generally to the field of working metal foils and, more particularly, to corrugating metal foils that exhibit low room temperature ductility, such as gamma-titanium aluminide (xcex3-TiAl) foils.
2. Background Information
Because of the light weight and desirable mechanical properties at elevated temperatures of xcex3-TiAl, significant research has been conducted regarding fabrication and producibility of honeycomb sandwich panels for use in high temperature aerospace applications. In order to produce xcex3-TiAl core sections for use in honeycomb panel construction, forming/corrugating thin foil strip is required. A significant problem with xcex3-TiAl is that it exhibits low room temperature ductility, which presents difficulties in forming it at room temperature. Moreover, xcex3-TiAl becomes more susceptible to surface oxidation when heated to high temperatures ( greater than 1400xc2x0 F.). In addition, when hot forming thin foils of any metal, rapid heat loss in the foil may occur during the forming process when the foil comes into contact with the machine forming tool faying surfaces (e.g., forming gears). This situation exacerbates the difficulties of consistently producing an end product that has the desired shape and is free of defects (e.g., cracks and altered surface grain structure). Furthermore, the environment around the forming tool area also may add to the foil forming/corrugation difficulties in regards to surface interstitial diffusion.
For the foregoing and other reasons, there is, accordingly, a need for a machine for, and a method of, corrugating metal foils that exhibit low room temperature ductility. In particular, there is a need for a machine, and a method, for corrugating such metal foils under conditions that ensure reliable production of a corrugated foil that is free of defects and has a desired end geometry.
The foregoing need is fulfilled, in accordance with the present invention, by a machine for corrugating metal foil strip lengths that includes an enclosure defining a chamber and a controllable heat source for heating the chamber. A gas or combination of gases may or may not be introduced into the chamber. At least one corrugation-forming tool set located in the chamber forms corrugations into the metal foil strip. Foil entrance feeder elements supply and guide the metal foil strip from outside the chamber into the chamber and to the tool set. A drive for the tool set is mounted outside the chamber and coupled to the tool set to actuate the tool set. Foil exit delivery elements guide the strip from the tool set and out of the chamber.
The heat source for the chamber maintainsxe2x80x94by using convection or radiation heating, or a combination thereofxe2x80x94a quasi isothermal temperature of the tool set and also heats the foil strip as it is guided to the tool set, such that when it is worked by the tool set it has sufficient ductility to be formed without cracking. Moreover, the heated tool set precludes any heat loss from the foil strip at the time of working that might alter its mechanical properties. The drives for the tool set are located outside the chamber where they are protected from the heat.
In some cases, the foil strip can be corrugated without heating the chamber to a temperature sufficiently high to oxidize the tool set, the foil strip, or both. When the machine and method involve temperatures in the chamber high enough to oxidize the tool set or foil strip, or both, a source supplying an inert gas to the chamber at a controlled gas flow rate may be used.
As explained more fully below, there are inherently significant gradients of heat along the length of the foil strip that resides at any given time between a supply roll of the foil stock and the delivery point of the corrugated foil strip after it leaves the heated chamber. On the incoming side of the chamber immediately outside of an opening in a wall of the chamber through which the strip enters the chamber, the cool incoming part of the strip is not heated enough to be subject to oxidation. While in the chamber, the inert gas prevents oxidation of the strip. The portion of the strip between the tool set and an exit opening from the chamber is progressively cooler near the exit opening, due to both heat loss by conduction along the strip to the cooler part of the strip outside of the chamber and to the cooler gases that are present near the walls of the chamber. Accordingly, when the strip leaves the chamber, it is no longer hot enough to be oxidized by the ambient air.
In preferred embodiments, the enclosure is double-walled and liquid-cooled so as to provide a large temperature gradient through the gas environment near the enclosure chamber walls (as well as through the chamber double walls). Those temperature gradients allow portions of the strip outside the chamber to remain at sufficiently low temperatures to avoid oxidation and to keep the outside of the enclosure at a relatively low temperature.
The enclosure may include partition walls forming a medial chamber and end sub-chambers on opposite ends of the medial sub-chamber and openings between the medial chamber and each sub-chamber through which the foil strip passes between the sub-chambers. This geometric arrangement of the entire chamber allows a foil strip to enter the medial sub-chamber from one end sub-chamber and to pass into the other end sub-chamber from the medial sub-chamber. The partition walls may be cooled with internal xe2x80x9cwater jackets.xe2x80x9d The tool set and the heating elements for heating the gas are located in the medial sub-chamber. The inert gas is supplied to the medial chamber. The partition walls of the medial chamber establish a temperature gradient between the inside of the medial chamber and the insides of the end sub-chambers. The inert gas passes from the medial sub-chamber through the openings in the partition walls into the end sub-chambers. The foil feeder elements and foil exit delivery elements guide the strip through the sub-chambers and/or through the medial chamber.
The foil feeder elements may include guide members within the chamber that form a serpentine delivery path for the strip so as to permit the strip to be heated before it reaches the tool set. Other suitable feeder elements include a guide chute supporting the strip along a path from the supply opening in a wall of the enclosure to the tool set. The guide chute provides a path for heat conduction along its length, so that the chute is relatively cool adjacent the wall of the enclosure and relatively hot near the tool set. The chute can be designed to establish a desired temperature gradient along its length. The foil strip, being in contact with the chute, exchanges heat with the chute and possesses a temperaturexe2x80x94and temperature gradientxe2x80x94close to that of the chute. Likewise, and with similar effect, the delivery elements may includexe2x80x94or consist ofxe2x80x94a guide chute supporting the strip along a path from the tool set to the exit opening in a wall of the enclosure.
The tooling in the enclosure may include a pre-form tool set that partially forms corrugations and a final tool set that fully forms the corrugations. Forming corrugations in two (or more) stages will affect the amount of foil springback. Given a similar final foil corrugation geometry, the strain rate during forming in each stage of a two-stage forming process will be less (for any given machine throughput) than if only a single-stage forming process is employed.
Various tool sets may be used in a machine according to the invention, such as:
1) A driven form gear having forming teeth and an idler form gear having forming teeth meshing with the forming teeth of the driven form gear and driven by the driven form gear.
2) A driven form gear having forming teeth, an idler pre-form gear having forming teeth meshing with the forming teeth of the driven form gear at a first location along the perimeter of the driven form gear and driven by the driven form gear, and an idler final form gear having forming teeth meshing with the forming teeth of the driven form gear at a second location along the perimeter of the driven form gear spaced apart from the first location and driven by the driven form gear.
3) A driven form gear having forming teeth, an idler form gear having forming teeth meshing with the forming teeth of the driven form gear, and a gear train coupling the driven form gear and the idler form gear so that both the driven and idler form gears are driven in rotation;
4) A pre-form tool set and a separate final tool set, each having a driven form gear having forming teeth, an idler form gear having forming teeth meshing with the forming teeth of the driven form gear, and a gear train coupling the driven form gear and the idler form gear so that both the driven and idler form gears are driven in rotation; the driven form gear of one of the tool sets is driven by the driven form gear of the other tool set.
5) A driven form gear having teeth defining cavities and a punch having a tooth substantially complementary in shape to the shape of the cavities. With a form gear/punch tool set, the drive includes a rotary drive that rotates the driven form gear and a reciprocating linear actuator driving the punch radially of the form gear. Preferably, the rotary drive rotates the form gear intermittently with a dwell period during which the punch forms a corrugation in the strip by deforming the strip into a cavity of the form gear. The punch may include a holder foot that engages an outgoing loop of a corrugation of the strip against the tip of the tooth of the form gear on the outgoing side of the cavity on each forming stroke of the tooth of the punch.
6) A pre-form tool set and a final tool set, both tool sets sharing a driven form gear having teeth defining cavities. The pre-form tool set includes a pre-form punch having a tooth partially complementary in shape to the shape of the cavities. The final pre-form tool set includes a final punch having a tooth substantially complementary in shape to the shape of the cavities. The final punch is spaced apart circumferentially of the form gear from the pre-form punch. The drive includes a rotary drive, preferably driven intermittently with a dwell period during actuation of the punches, rotating the driven form gear and a reciprocating linear actuator driving each punch radially of the form gear. Each punch may have a holder foot that engages an outgoing loop of a corrugation of the strip against the tip of the tooth of the form gear on the outgoing side of the cavity on each forming stroke of the tooth of the punch.
The foregoing description has outlined rather broadly some features and advantages of the present invention. The detailed description of embodiments of the invention that follows will enable the present invention to be better understood and the present contribution to the art to be more fully appreciated. Those skilled in the art will recognize that the embodiments may be readily utilized as a basis for modifying or designing other structures and methods for carrying out the purposes of the present invention. All such structures and methods are intended to be included within the spirit and scope of the invention as set forth in the appended claims.